In a letter released on Monday, the former national security officials said the US president's actions represented a "political litmus test" that "weakened" the country.

"Former government officials have the right to express their unclassified views on what they see as critical national security issues without fear of being punished for doing so," they said.

New signatories to the statement, which was initially issued last week by 15 former directors and deputy chiefs of the CIA and Office of Director of National Intelligence, include former political appointees and career civil servants. They worked under both Democratic and Republican presidents.

Among the most prominent individuals to sign a new version of the statement released on Monday by senior officials from the George W Bush and Obama administrations are former State Department and National Security Counsel lawyer John Bellinger, former Deputy Secretaries of State Anthony Blinken and William Burns, former Undersecretaries of State Nicholas Burns, Wendy Sherman and Thomas Pickering.

John Brennan says he's prepared to take legal action against the presidentCredit:
AFP

The White House has defended Mr Trump's decision to revoke security clearances, saying in Mr Brennan's case it was down to his "erratic conduct" and "unfounded, outrageous" allegations about the administration.

While the signatories stressed that they may not agree with all of Mr Brennan's public rebukes of Mr Trump, the statement reads, they believe that "the country will be weakened if there is a political litmus test applied" before expert former officials are allowed to voice their views.

The security clearance of several other former intelligence officials, including James Clapper, the former director of national intelligence, and ex-FBI director James Comey are also "under review".

President Trump has taunted the former CIA chief - daring him to take legal action against himCredit:
Bloomberg

Former intelligence officials usually keep their security clearance but rarely use it, sometimes providing advice to successors.

Mr Brennan, one of the president's most vocal critics, has claimed the investigation into Russian election meddling was "well-founded" and suggested the Trump campaign is guilty of collusion.

During an interview on for US network NBC, Mr Brennan said he is considering taking legal action against Mr Trump.

"I am going to do whatever I can personally to try to prevent these abuses in the future. And if it means going to court, I will do that."

Mr Brennan added that his reputation was "a small price to pay" to prevent Mr Trump from taking the same punitive actions against others in future.

I hope John Brennan, the worst CIA Director in our country’s history, brings a lawsuit. It will then be very easy to get all of his records, texts, emails and documents to show not only the poor job he did, but how he was involved with the Mueller Rigged Witch Hunt. He won’t sue!

Mr Trump taunted the former CIA chief on Monday, tweeting: "I hope John Brennan, the worst CIA Director in our country's history, brings a lawsuit. It will then be very easy to get all of his records, texts, emails and documents to show not only the poor job he did, but how he was involved with the Mueller Rigged Witch Hunt. He won’t sue!

Everybody wants to keep their Security Clearance, it’s worth great prestige and big dollars, even board seats, and that is why certain people are coming forward to protect Brennan. It certainly isn’t because of the good job he did! He is a political “hack.”

Mr Trump also suggested that other security officials were only supporting Mr Brennan because they wanted to keep their own clearances, which are worth "big dollars".

"Everybody wants to keep their Security Clearance, it’s worth great prestige and big dollars, even board seats, and that is why certain people are coming forward to protect Brennan. It certainly isn’t because of the good job he did! He is a political “hack.”